Load compensated control system



Sept. 12, 1939. H. G. WASSERLEIN. JR 2,172,487

LOAD COMPENSATED CONTROL SYSTEM Filed Dec. 19, 1935 Patented Sept. 12,1939 UNITED STATES PATENT OFFICE 2,172,487 7 LOAD COMPENSATED CONTROLSYSTEM of Delaware Application December 19, 1935, Serial No. 55,216

16 Claims.

This invention relates to control systems of the type disclosed inapplication Ser. No. 673,236, filed by Lewis L. Cunningham on May 27,1933. In the type of system disclosed in the abovereferred toapplication, the condition being controlled is maintained withinpredetermined limits, the control point and consequently the value ofthe condition varying between these limits in accordance with the loadon the system. When such a system is used, for example, as a temperaturecontrol system for a furnace, the control point and consequently thetemperature of the furnace remains constant as long as the load on thefurnace remains constant, but as the load on the furnace varies, thecontrol point and consequently the temperature of the furnace varieswithin predetermined limits. Specifically, an increase in the load onthe furnace causes a lowering of the control point and a decreasing ofthe temperature of the furnace, and vice versa, a decrease in the loadon the furnace causes an increase in the temperature of the furnace anda raising of the control point. However, the control point andconsequently the temperature of the furnace is maintained within thelimits, set for the control system.

In industrial processes, it is desirable to maintain conditions ortemperatures substantially constant. If the differential of the systembe decreased to maintain these conditions or temperatures substantiallyconstant, hunting is likely to occur. This hunting may be eliminated bywidening of the differential or control but this widening of thedifferential does not maintain accurately the temperature or conditionat a substantially constant value.

It is therefore the prime object of this inventlon to provide a systemutiliz'ng a narrow differential of control whereby the temperature orconditions to be controlled may be maintained substantially constant andwherein hunting of the system is prevented.

In carrying out this object of the invention, I contemplate the use ofchanges in the current flow throughout the various connections in thissystem for compensating the control system of the above-referred toapplication whereby the control system may be operated within narrowdifferentials and whereby changes in load are compensated for so thathunting" of the control system is prevented.

It is therefore an object of this invention to provide a compensatedcontrol system wherein changes in. the current flow in such a controlsystem is utilized for compensating changes in load to maintain asubstantially constant condition without the occurrence of hunting.

Other objects and advantages will become apparent to those skilled inthe art upon reference to the accompanying specification, claims anddrawing, in which drawing is diagrammatically illustrated the preferredform of my invention.

Although this invention is of general utility and may be utilized in anytype of control system, I have shown it as a temperature control system,having a temperature responsive device generally designated at H! forcontrolling or positioning a valve l5. For purposes of illustration, thevalve l5 may be operated to control a supply fuel to an industrialfurnace (not shown) and the temperature responsive device I0 may respondto the temperature within the funace.

The temperature respons've device in is s" xwn to comprise a bimetallicelement II, carrying slider i 2 which is adapted to slide across a potentiometer coil l3. Upon an increase in furnace temperatures, the sliderI2 is moved to the left with respect to the potentiometer coil l3 in thedirection indicated by the character H. Upon a decrease in furnacetemperature, the slider I2 is moved to the right with respect to thepotentiometer .coil iii in the direction indicated by the character C.

The valve I5 is shown to control the supply of fuel from a supply pipe[6 to a pipe ll leading to a burner in the furnace. The valve i5 may beoperated by a proportion'ng motor of the type shown and described in theabove referred to application but for purposes of illustration in thisapplication, I have shown the valve l5 to beoperated by a valve stem l8which is connected by a pin to one end of a pitman l9. The other end ofpitman I 9 is connected to a crank pin carried by a gear 20 and a crankdisc 2i. The gear 20 may be mounted on a shaft 22 and the crank disc 2|may be mounted on a shaft 22'. The gear 20 is driven through a reductiongear train' 23 by motor rotors 24 and 25, the motor rotors beingcontrolled by field windings 26 and 21, respectively. The arrangement issuch that when the field winding 26 is energized, the valve I5 is movedtowards an open position and when the field winding 21 is energized. thevalve [5 is moved towards a closed position.

The shaft 22 carries a bevelled gear 28 which meshes with anotherbevelled gear 29 which in turn carries abutment fingers 30 and 31. Uponextreme clockwise movement of the beveled gear 29, the abutment finger30 is adapted to engage a contact 32 to break contact between thecontact 32 and a contact 33.

Likewise, upon extreme counterclockwise movement of bevelled gear 29,the abutment finger 3| is adapted to engage a contact 34 to breakcontact between the contact 34 and a contact .35. The contacts 32, 33,34 and 35 form limit switches, the operation of which will be pointedout more fully hereafter. The shaft 22 carries a bevelled gear 36 whichmeshes with a bevelled gear 31' which in turn carries a slider 38. Theslider 38 is adapted to slide across a potentiometer coil 39, thearrangement being such that when the valve I is moved towards an openposition, the slider 38 is moved to the left with respect to thepotentiometer coil 39 and when the valve I5 is moved towards a closedposition, the slider 38 is moved towards the right.

Coils 4| and 42 control the operation of an armature .43 which isconnected by means of a spring 44 to a pivoted switch arm 45. Switch arm45 is adapted to engage spaced contacts 46 and 41, the arrangement beingsuch that when the coil 4| is energized more than the coil 42, theswitch arm 45is moved into engagement with the contact 46 and when thecoil 42 is energized more than the coil 4|, the switch arm 45 is movedinto engagement with the contact 41. When the coils 4| and 42 areequally energized, the switch arm 45 is maintained midway between thecontacts 46 and 41 in the position shown in the drawing. This inventionalso contemplates the use of coils 48 and 49 which control the operationof an armature 50. The armature 58 is connected by a spring 5| to apivoted switch arm 52 which is adapted to engage spaced contacts 53 and54. When the coil 49 is energized more than the coil 48, the switch arm52 is moved into engagement with the contact 54 and when the coil 48 isenergized more than the coil 49, the switch arm 52 is moved intoengagement with .the contact 53. When the coils 48 and 49 are equallyenergized, the switch arm 52 is moved to a position midway between thecontacts 53 and 54 in the position shown in the drawing.

This invention contemplates the use of compensators generally designatedat 56 and 51. The compensator 56 is shown to comprise a bimetallicelement 58 suitably mounted on a block 59. Suitably mounted on andinsulated from the bimetallic element 58 is a slider 60 which is adaptedto slide across a resistance 66. Located in close proximity to thebimetallic element 58 is a heater coil 6|, the arrangement being suchthat when the heater coil 6| is energized, the bimetallic element 58 isflexed to move the slider 66 towards the right with respect to theresistance 66 in the direction indicated by the character H. Thestructure and mode of operation of compensator 51 is the same as that of56 and comprises a bimetallic element 62, mounted on a block 63 andcarrying, in an insulated manner, a slider 64 which is adapted to slideacross a resistance 61. The bimetallic element 62 is likewise providedwith an electric heating coil 65, the arrangement bein such that uponenergization of the electric heating element 65, the slider 64 is movedto the left with respect to the resistance 61 in the direction indicatedby the character H. It is evident that some time will-elapse before achange in the heating efiect of the heater elements 6| and 65 is felt bythe thermostatic elements 58 and 62. In other words, a time lag will bepresent between the change in current flow through the heater elements6| and 85 and the movement of the sliders 60 and 64 with respect totheir resistances 66 and 61.

Leading from some source of power (not shown) are line wires 19 and 1|.A primary 12 of a step-down transformer 13, having a secondary 14 isconnected across the line wires and 1| One end of the secondary 14 isconnected by wires and 16 to the left-hand ends of coils 48 and 4|. In alike manner, the right-hand end of secondary 14 is connected by wires 11and 18 to the right-hand ends of coils 49 and 42. The adjacent ends ofthe coils 48 and 49 and the coils 4| and 42 are connected together. Theleft-hand end of coil 4| is connected by a protective resistance 19 andwires 80 and 8| to one end of heat ing coil 6| of the compensator 56.The other end of heating coil 6| is connected to the slider 66. In alike manner, the right-hand end of coil 42 is connected by a protectiveresistance 82 and wires 83 and 84 to one end of the heating coil 65 ofthe compensator 51. The other end of the heating coil 65 is connected tothe slider 64. The resistance 66 associated with the compensator 56 isconnected by a wire 85 to the left-hand end of potentiometer coil l3 andthe resistance 61 associated with the compensator 51 is connected by awire 86 to the right-hand end of potentiometer coil l3. The junction ofwires 88 and 8| is connected by a wire 81 to the left-hand end ofbalancing potentiometer coil 39 and in a like manner, the junction ofwires 83 and 84 is connected by a wire 88 to the right-hand end of thebalancing potentiometer coil 39. The junction of coils 4| and 42 isconnected by wires 89, 90 and 9| to the slider |2 associated with thepotentiometer coil l3 and the slider 38 associated with the balancingpotentiometer coil 39.

The contact 46 is connected by a wire 93 to a small number of turns ofcoil 4| and the contact 41 is connected by a wire 94 to a small numberof turns of the coil 42. Switch arm 45 associated with the contacts 46and 41 is connected by a wire 95 to the junction of coils 48 and 49. Theswitch arm 52 is connected by a wire 96 to the line wire 10 and thecontact 53 cooperating with the switch arm 52 is connected by a wire 91to the contact 35 of the limit switch. In a like manner, the contact 54is connected by wire 98 to the contact 32 of the other limit switch.Contact 34 of the first limit switch is connected by a wire 99 to oneend of field winding 21 and the contact 33 of the other limit switch isconnected by a wire I80 to one end of field winding 26. The other endsof field windings 26 and 21 are connected together and by a wire |6| tothe other line wire 1|.

By reason of the above wiring connections, it is seen that the. coils 4|and 42 and the coils 48 and 49 are connected across the secondary 14 bymeans of the wires 15, 16, 11 and 18. It is also seen that a shuntcircuit for the coil 4| is provided by the protective resistance 19, the

/ wires 80 and 8|, the heating coil 6|, slider 66,

resistance 68, wire 85, potentiometer coil l3,

slider l2 and wires 90 and 89. In a like manner,

ing potentiometer coil 39, slider 38, and wires 9| and 89. It is alsoseen that when the switch arm 45 is moved into engagement with thecontact 41, a shunt circuit for the coil 49 is provided through wire 18,a small number of turns of coil 42, wire 94, contact 41, switch arm 45and wire 95. A shunt circuit for the coil 48 is provided when the switcharm 45 is moved into engagement with the contact 46 through wire 16, asmall number of turns 01' coil 4|, wire 93, contact 46, switch arm 45and wire 95.

With the parts in the position shown in the drawing, the slider |2 is inthe mid-position with respect to the potentiometer coil I3, the valve I5is in mid-position and consequently the slider 38 is in the midpositionwith respect to the balancing potentiometer coil 39. Likewise, thesliders 69 and 64 are located in their mid positions with respect totheir resistance coils 66 and 61. By reason of these midpositions, theflow of current through the above outlined shunt circuits for the ,coils4| and 42 is equal and consequently, the coils 4| and 42 are equallyenergizedto maintain the switch arm 45 in the mid position with respectto the contacts 46 and 41. With the switch arm 45 in this mid position,the shunt circuits for the coils 48 and 49 are broken and therefore thecoils 48 and 49 are equally energized to maintain the switch arm 52 midway between the contacts 53 and 54.

Omitting for the time being the action of compensators 56 and 51, uponmovement of the slider 2 to the right with respect to the potentiometercoil l3 by reason of a decrease in furnace temperature, the flow ofcurrent through the shunt circuit for the coil 42 is increased, therebydecreasing the flow of current through the coil 42 to decrease theenergization thereof. This right-hand movement of slider l2 alsoincreases the resistance in the shunt circuit for the coil 4| todecrease the flow of current through this shunt circuit and therebyincrease the flow of current through the coil 4|. Therefore, by reasonoi the right-hand movement of slider.:'-|2, the coilf4l is energizedmore than the coil 42 and the switch arm 45 is moved into en- Easementwith the contact 46. Movement of switch armf" into engagement with thecontact 46 comple't'is thelshunt circuit for the coil 48 to decrease thehow of current through the coil 48 whereby the coil 49 is energized morethan the call". This unbalanced relationship of the coils 49 and 48causes movement of switch arm 52 into engagement with th contact 54 tocomplete a circuit from the line wires 18 through wire 96, switch arm52, contact 54, wire 98, limit switch contacts 32 and 33, wire I66,field winding 26 and wire |6|, back to the other line wire 1|.Completion of this circuit causes energization of field winding 26 tocause movement of valve l5 towards an open position.

Movement of the valve l5 towards an open position causes left-handmovement of slider 36 with respect to the balancing potentiometer coil39. This left-hand movement of slider 36 decreases the resistance in thesecond shunt circuit for the coil 4| and increases the resistance in thesecond shunt circuit for the coil 42. Therefore, the current flowthrough the shunt circuit for the coil 4| is increased and the currentflow through the coil 4| is decreased. Likewise, the current flowthrough the shunt circuit for the coil 42 is decreased and the currentflow through the coil 42 is increased. By reason 01' this left handmovement oi slider 36, the

coil 42 is caused to be energized more than the coil 4|, it beingremembered that the coil 4| was energized more than the coil 42 by theright-hand movement of slider |2 with respect to the potentiometer coil|3. When the slider 38 has moved suihciently far to the left withrespect to the balancing potentiometer coil 39 so as to cause equalenergization of coils 4| and 42, the switch arm 45. is moved out ofengagement with the contact 46. This causes breaking of the shuntcircuit for the coil 48 to cause equal energization of coils 48 and".This rebalancing of coils 48 and 49 causes movement of switch arm 52 outof engagement with contact 54 to break the circuit through the fieldwinding 26 to stop further opening movement of the valve l5.

Upon an increase in temperature within the furnace, the slider |2is'moved to the left with respect to the potentiometer coil |3 todecrease the resistance in the shunt circuit for the coil 4| and therebydecrease the current flow through the coil 4| to give a consequentdecrease in the energization thereof. Left-hand movement of slider l2places more resistance in the shunt circuit for the coil 42 to'decreasethe'flow of current through this shunt circuit and thereby increase theflow of current through the coil 42 to increase the energizationthereof. By reason of this unbalanced relationship of coils 4| and 42,the switch arm 45 is moved into engagement with the contact 41 tocomplete the shunt cir-- cuit for the coil 49 to decrease theenergization of coil 49. By reason of this unbalanced rela tionship ofcoils 40 and 49, the switch arm 52 is moved into engagement with thecontact 53 to complete a circuit from the line wire 10 through wire 96,switch arm 52, contact 53, wire 91, contacts 35 and 34, wire 99, fieldwinding 21 and wire back to the other line wire 1|. Completion. of thiscircuit causes energization of field winding 21 to move the valve Itowards a closed position.

Movement of valve |5 towards a closed position causes right-handmovement of slider 38 with respect to the balancing potentiometer coil39. This right-hand movement increases the resistance in the secondshunt.circuit for the coil 4| todecreasethe current flow through theshunt circuit and increase the current flow through the coil 4|. Thisright-hand movement of slider 38 subtracts resistance from the shuntcircuit for the coil 42 to increase the flow of current through theshunt circuit to decrease the flow of current through the coil 42.Therefore, the right hand movement of slider 38 increases theenergization of coil 4| and decreases the energization of coil 42, itbeing remembered that the energization of coil 42 was increased withrespect to the energization of coil 4| by the left-hand movement ofslider |2 with respect to the potentiometer coil 3.

When the slider 38 has been moved suiliciently far to the right torebalance the coils 4| and 42, the switch arm 45 is moved out ofengagement with the contact 41 to break the shunt circuit for the coil49 whereby the coils 48 and 49 again become equally energized. Due tothis rebalancing of the-coils 48 and 49, the switch arm 52 is moved outof engagement with switch arm 53 to the mid position shown in the,drawing to break the circuit through the field winding 21 to preventfurther closing movement of the valve |5.

In this manner, the valve 5 is caused to assume a position correspondingto the position of the slider l2 with respect to the potentiometer coilit. Therefore, upona decrease in furnace temperature, the valve i5 ismoved towards an open position in a proportionate amount with respect tothe decrease in furnace temperature to attempt to restore the furnacetemperature back to normal. Likewise, an increase in furnace temperaturecauses a proportionate closing of the valve iii in an attempt to restorethe furnace temperature to normal.

If the parts be so adjusted that the slider l2 is maintained in a midposition with respect to the potentiometer coil l3 and the valve inmaintained in a mid position for a given load condition in the furnace,a constant temperature may be maintained within the furnace as long asthe load on the furnace remains constant. Movement of slider l2 to theright in response to a decrease in furnace temperature will causeopening movement of the valve l5 to restore the temperature to normaland when the temperature is restored to normal, the slider i? will againassume the mid position. If however, the load on the furnace isincreased so as to demand more heat to maintain a given temperaturevalue, the valve l5 therefore should be opened further, and in order tomaintain the valve id in this further open position, the slider l2 mustassume a position to the right with respect to the mid position of thepotentiometer coil it. Therefore, the temperature within the furnace isnot maintained at the desired value but at some value lower than thedesired value, this decrease in the temperature maintained in thefurnace being caused by the increased load on the furnace. In a likemanner, a decrease in the load on the furnace will cause the existenceof an increased temperature in the furnace slightly above the desiredvalue. Therefore, with the system described immediately above, thetemperature within the furnace cannot be maintained constant but can bemaintained only within predetermined limits.

As pointed out above, if the limits are made sufficiently narrow tomaintain a substantially constant temperature within the furnace, thesystem is liable to hunt" and to decrease this hunting action of thesystem it has been in the past necessary to widen the differential ofoperation of the system. However, this widened differential ofoperatioii does not afford sufficiently accurate control when used inindustrial processes.

In order to maintain a substantially constant temperature within thefurnace and yet prevent hunting", I have provided the compensators 56and 51? in the first shunt circuits for the coils ll and 62. Upon anincrease in load on the furnace which causes a decrease in temperature,the slider i2 is moved to the right with respect to the potentiometercoil iii to decrease the resistance in the shunt circuit for the coil 42and to increase the resistance in the shunt circuit for the coil All.This causes increasing of the energization of coil ll and decreasing ofthe energization of the coil d2. Since the shunt circuit for the coil 62passes through the heater 65 of the compensator hi and since the currentflow through this shunt circuit is increased by this right-hand movementor" slider 52, the heating coil 65 is energized to a greater extent tocause flexing of the bimetal G2 to move the slider 64! to the left withrespect to the resistance coil till to further reduce the re sistance inthe shunt circuit for the coil 62. By reason of this left-hand movementof slider 66, the energization of coil M is decreased further than ifthe compensator 5'! were not contained in this shunt circuit. Likewise,the movement of slider E2 to the right with respect to the potentiometercoil l3 decreases the flow of current through the shunt circuit for thecoil 4| to decrease the current flow through the heating coil St toallow the bimetallic element 58 to slide the slider 60 to the left withrespect to the resistance 66. This left-hand movement of slider (ill byreason of the decreased current fiow through the shunt circuit for thecoil 4i adds more resistance to the shunt circuit to further decreasethe current flow therethrough and thereby further increase theenergization of the coil ll. Since the coil fill is energized to agreater extent and the coil 4'2 is energized a lesser extent by reasonof compensators 56 and till, greater movement of valve 55 towards openposition with consequent movement of slider til to the left with respectto the balancing potentiometer coil 39 is required to rebalance thecoils ill and 42. Therefore, with the slider i2 moved to a givenposition with respect to the potentiometer coil it, the valve i5 ismoved to a further open position to supply additional heat to thefurnace. In this manner, as the slider 62 deviates from the center ofpotentiometer coil 03, the valve i2 is not moved exactly proportionateto the movement of slider l2 but moved to a greater extent and thisgreater movement of the valve l5 causes an increased supply of heat tothe furnace to maintain the temperature of the furnace at the desiredvalue regardless of the increase in load.

In a like manner, upon a decrease in load on the furnace, the slider i2moves to the left with respect to the potentiometer coil iii to increasethe energization of coil 42 and decrease the energization of coil 49. Byreason of the compensators 56 and 57 being located in the shunt circuitsfor the coils 4i and 42, the coil 42 is energized to a greater extentand the coil 4! is energized to a lesser extent than if the compensators56 and bi were not present. Therefore, upon a decrease in load on thefurnace, the valve [5 is moved towards a closed position to supply lessheat to the furnace whereby the furnace temperature is maintained at asubstantially constant value regardless of the effects of this decreasein load on the furnace. As pointed out above the adjustment of thecompensating resistances 66 and 61 lags somewhat behind the change incurrent flow through the heater ele ments 6i and 65 so that theadditional positioning or resetting of the valve I5 is delayed somewhatand takes place rather gradually which effectively prevents hunting. Theamount and rate of additional positioning or resetting of the valve i5is dependent upon how far the slider l2 of the control potentiometerdeviates from the desired normal position. Accordingly, when the slider12 of the control potentiometer deviates from the mid position shown,the valve 15 is moved quite rapidly a corresponding amount and then issubsequently gradually moved an additional amount, the rate and amountof additional movement of the valve l5 being dependent upon the amountof deviation of the slider l2 of the control potentiometer from the midposition.

From the above it is seen that I have provided a control system whereinchanges in load conditions are compensated for to maintain thetemperature or condition to be controlled at a substantially constantvalue and wherein hunting of such a control system is prevented.

Although I have disclosed one form of my invention, other forms thereofmay become apparent to those skilled in the art and consequently thisinvention is to be limited only by the scope of the appended claims andprior art.

I claim as my invention:

1. In a temperature control system, the combination of temperaturecontrol means, means responsive to the temperature to be controlled, anadjustable potentiometer operated by said temperature responsive means,electric means in control of said temperature control means, connectionsbetween said adjustable potentiometer and said electric means, thearrangement being such that adjustment of said adjustable po tentiometerby said temperature responsive means varies the current flow throughsaid connections to adjust said temperature control means, variableresistance means in said connections, and thermo-electric meansresponsive to the current flow through said connections for adjustingsaid variable resistance means whereby the temperature control means isadjusted with respect to said adjustable potentiometer.

2. In a control system, the combination of means for controlling acondition, means responsive to the condition to be controlled, variableresistance means operated by said condition responsive means, electricmeans in control of said condition controlling means, connectionsbetween said variable resistance means and said electric means, thearrangement being such that operation of said variable resistance meansby said condition responsive means varies the cur rent flow through saidconnections to adjust said condition controlling means, variableresistance means in said connections, thermostatic means for operatingsaid last mentioned variable resistance means, and heating means also insaid connections for operating said thermostatic means, said heatingmeans being variably ener gized in accordance with the current flowthrough said connections to vary the current flow therethrough wherebysaid condition controlling means is adjusted with respect to said firstmentioned variable resistance means.

3. In a control system, the combination of means for controlling acondition, electric means for operating said condition controllingmeans, current controlling means connected to said electric means andresponsiye to the condition to be controlled for varying the currentflow to said electric means, the arrangement being such that thecondition to be controlled is maintained within predetermined limits,and thermo-electric means responsive to the current flow to saidelectric means'for additionally varying the current flow to saidelectric means whereby said condition to becontrolled is maintainedwithin narrower limits.

4. In a control system, the combination of means for controlling acondition, electric means for operating said condition controllingmeans, current controlling means operated in response to changes in thecondition to be controlled and connected to said electric means forvarying the current flow to said electric means, the arrangement beingsuch that the operation of said con dition controlling means is variedin accordance with changes in the condition being controlled to maintainthe condition to be controlled within predetermined limits, andthermo-electric means responsive to the current flow to the electricmeans for controlling the electric means to maintain the condition to becontrolled within narrower limits.

5. In combination, a device to be positioned in a plurality of positionsto control the value of a condition, electrical resistance means variedin accordance with changes in the value of the condition to becontrolled and having a desired resistance value corresponding to adesired normal value of the condition, electrical relay means in controlof said device, follow up means for said relay means operated by saiddevice, connections between said relay means and said resistance means,said resistance means varying the current flow through said connectionsto operate said relay means for positioning said device in accordancewith changes in-the resistance value of the resistance means whereby thevalue of the condition to be controlled is maintained within certainlimits, and means responsive to the current flow through saidconnections additionally to position said device for causing said deviceto assume a diiferent position with respect to the resistance value ofthe resistance means to maintain the condition to be controlled withinnarrower limits.

6. In combination, a device to be positioned in a plurality of positionsto control the value of a condition, electrical resistance means variedin accordance with changes in the value of the condition to becontrolled and having a desired resistance value corresponding to adesired normal value of the condition, electrical relay means in controlof said device, follow up means for said relay means operated by saiddevice, connections between said relay means and said resistance means,said resistance means varying the cur rent flow through said connectionsto operate said relay means for positioning said device in accordancewith changes in the resistance value of the resistance means whereby thevalue of the condition to be controlled is maintained within certainlimits, and means responsive to the current flow through saidconnections for also operating said relay means additionally to positionsaid device for causing said device to assume a different position withrespect to the resistance value of the resistance means to maintain thecondition to be controlled within narrower limits.

7. In combination, a device to be positioned in a plurality of positionsto control the value of a condition, electrical resistance means variedin accordance with changes in the value of the condition to becontrolled and having a desired resistance value corresponding to adesired normal value of the condition, electrical relay means in controlof said device, follow up means for said relay means operated by saiddevice, connections between said relay means and said resistance means,said resistance means varying the current flow through said connectionsto operate said relay means for positioning said device in accordancewith changes in the resistance value of the resistance means whereby thevalue of the condition to be controlled is maintained within certainlimits, and thermo-electric means responsive to the current flow throughsaid connections for also operating said relay means additionally toposition said device.

8. In combination, a device to be positioned in a plurality of positionsto control the value of a condition, electrical control resistance meansvaried in accordance with changes in the value of the condition to becontrolled and having a desired resistance value corresponding to adesired normal value of the condition, electrical relay means in controlof said device, follow up means for said relay means operated by saiddevice, connections between said relay means and said resistance means,said resistance means varying the current flow through said connectionsto operate said relay means for positioning said device in accordancewith changes in the resistance value of the control resistance meanswhereby the value of the condition to be controlled is maintained withincertain limits, other variable resistance means associated with saidrelay means for operating also said relay means, and means responsive tothe current flow through said connections for operating said variableresistance means additionally to position said device for causing saiddevice to assume a difierent position with respect to the resistancevalue of the control resistance means to maintain the condition to becontrolled within narrower limits.

9. In combination, a device to be positioned in a plurality of positionsto control the value of a condition, electrical resistance means variedin accordance with changes in the value of the condition to becontrolled and having a desired resistance value corresponding to adesired normal value of the condition, electrical relay means in controlor said device, follow up means for said relay means operated by saiddevice, connections between said relay means and said resistance means,said resistance means varying the current flow through sa d connectionsto operate said relay means for positioning said device in accordancewith changes in the resistance value of the resistance means whereby thevalue of the condition to be controlled is maintained within certainlimits, other variable resistance means associated with said relay meansfor operating also said relay means, and thermo-electric meansresponsive to the current flow through said connections for operatingsaid variable resistance means additionally to position said device.

10. In combination, a device to be positioned in a plurality ofpositions to control the value of a condition, control electricalresistance means varied in accordance with changes in the value of thecondition to be controlled and having a desired resistance valuecorresponding to a desired normal value of the condition, electricalrelay means in control of said device, balancing electrical resistancemeans operated by said device, connections between said controlresistance means, said balancing resistance means and said relay means,said resistance means varying the current fiow through said connectionsto operate said relay means for positioning said device in accordancewith changes in the condition to be controlled whereby the condition tobe controlled is maintained Within certain limits, and means responsiveto the current flow through said connections additionally to positionsaid device for causing said device to assume a different position withrespect to the resistance value of the resistance means to maintain thecondition to be controlled within narrower limits.

11. In combination, a device to be postioned in a plurality of positionsto control the value of a condition, control electrical resistance meansvaried in accordance with changes in the value of the condition to becontrolled and having a desired resistance value corresponding to-adesired normal value of the condition, electrical relay means in controlof said device, balancing electrical resistance means operated by saiddevice, connections between said control resistance means, saidbalancing resistance means and said relay means, said resistance meansvarying the current flow through said connections to operate said relaymeans for positioning said device in accordance with changes in thecondition to be controlled whereby the condition to be controlled ismaintained within certain limits, and means responsive to the currentflow through said connections for also operating said relay meansadditionally to position said device for causing said device to assume adifferent position with respect to the resistance value of theresistance means to maintain the condition to be controlled withinnarrower limits.

12, In combination, a device to be positioned in a plurality ofpositions to control the value of a condition, control electricalresistance means varied in accordance with changes in the value of thecondition to be controlled and having a desired resistance valuecorresponding to a desired normal value of the condition, electricalrelay means in control of said device, balancing electrical resistancemeans operated by said device, connections between said controlresistance means, said balancing resistance means and said relay means,said resistance means varying the current flow through said connectionsto operate said relay means 'for positioning said device in accordancewith changes in the condition to be controlled whereby the condition tobe controlled is maintained within certain limits, other variableresistance means associated with said relay means for operating alsosaid relay means, and means responsive to the current flow through saidconnections for operating said variable resistance means additionally toposition said device for causing said device to assume a differentposition with respect to the resistance value of the control resistancemeans to maintain the condition to be controlled within'narrower limits.

13. In combination, a device to be positioned in a plurality ofpositions to control the value of a condition, control electricalresistance means varied in accordance with changes in the value of thecondition to be controlled and having a desired resistance valuecorresponding to a desired normal value of the condition, electricalrelay means in control of said device, balancing electrical resistancemeans, operated by said device, connections between said controlresistance means, said balancing resistance means and said relay means,said resistance means varying the current flow through said connectionsto operate said relay means for positioning said device in accordancewith changes in the condition to be controlled whereby the condition tobe controlled is maintained within certain limits, other variableresistance means associated with said relay means for operating alsosaid relay means, and thermo-electric means responsive to the currentflow through said connections for operating said variable resistancemeans additionally to position said device.

14. In combination, a device to be positioned in a plurality ofpositions to control the value of a condition, a control potentiometeradjusted in accordance with changes in the value of the condition to becontrolled and having a desired adjustment corresponding to the desirednormal value of the condition, a normally balanced relay includingseries connected relay coils in control of said device, connectionsbetween said relay coils and said control potentiometer, said con-= trglpotentiometer varying the current flow through said connections tounbalance said relay for positioning said device in accordance with theadjustment of the control potentiometer whereby the condition to becontrolled is maintained within certain limits, and means responsive tothe current flow through said connections additionally to position saiddevice.

, 15. In combination, a device to be positioned in a plurality ofpositions to control the value of a condition, a control potentiometeradjusted in accordance with changes in the value of the condition to becontrolled and having a desired adjustment corresponding to the desirednormal value of the condition, a normally balanced relay includingseries connected relay coils in control of said device, connectionsbetween said relay coils and said control potentiometer, said controlpotentiometer varying the current flow through said connections tounbalance said relay for positioning said device in accordance with theadjustment of the control potentiometer whereby the condition to becontrolled is maintained within certain limits, and means responsive tothe current flow through said connections for also unbalancing saidrelay means additionally to position said device.

16. In combination, a device to be positioned in a plurality ofpositions to control the value of a condition, a control potentiometeradjusted in accordance with changes in the value of the condition to becontrolled and having a desired adjustment corresponding to the desirednormal value of the condition, a normally balanced relay includingseries connected relay coils in control of said device, connectionsbetween said relay coil and said control potentiometer, said controlpotentiometer varying the current flow through said connections tounbalance said relay for positioning said device in accordance with theadjustment of the control potentiometer whereby the condition to becontrolled is maintained within certain limits, variable resistancemeans associated with said relay coils for unbalancing said relay, andmeans responsive to the current flow through said connections foroperating said variable resistance means additionally to position saiddevice.

HENRY G. WASSERLEIN, JR.

